Liquid-activated quantum emission from native hBN defects for nanofluidic sensing

Abstract

Nanostructures made of two-dimensional (2D) materials have become the flagship of nanofluidic discoveries in recent years 1,2 .By confining liquids down to a few atomic layers, anomalies in molecular transport 3-5 and structure 6, 7 have been revealed.Currently, only indirect and ensemble averaged techniques have been able to operate in such extreme confinements, as even the smallest molecular fluorophores are too bulky to penetrate state-of-the-art single-digit nanofluidic systems 8 . This strong limitation calls for the development of novel optical approaches allowing for the direct molecular imaging of liquids confined at the nanoscale. Here, we show that native defects present at the surface of hexagonal boron nitride 9 (hBN) -a widely used 2D material -can serve as probes for molecular sensing in liquid, without compromising the atomic smoothness of their host material. We first demonstrate that native surface defects are readily activated through interactions with organic solvents and confirm their quantum emission properties. Vibrational spectra of the emitters suggest that their activation occurs through the chemisorption of carbon-bearing liquid molecules onto native hBN defects. The correlated activation of neighboring defects reveals single-molecule dynamics at the interface, while defect emission spectra offer a direct readout of the local dielectric properties of the liquid medium. We then harvest these effects in atomically smooth slit-shaped van der Waals channels, revealing molecular dynamics and increasing dielectric order under nanometre-scale confinement. Liquid-activated native defects in pristine hBN bridge the gap between solid-state nanophotonics and nanofluidics and open up new avenues for nanoscale sensing and optofluidics. MainFluorescent defect centers in wide band-gap materials like diamond, silicon carbide or boron nitride have received increasing attention in recent years as they allow to probe nanoscale matter through interactions with their dipoles and electron spins 10,11 . In particular, a variety of point defects with energy levels well within the 6 eV band gap of hexagonal boron nitride (hBN) were

Nanofluidics at the crossroads

“…This imaging technique notably showed how proton diffusion is considerably slowed down near a chemically active surface. The same technique was later used to image proton transport, this time in a confined environment consisting of a nanofluidic slit (height of 1 − 2 nm) 55 . Such setups could allow direct access to the molecular dynamics inside nanochannels, particularly in cases where surfaces processes play an important role.…”

Label-free imaging of DNA interactions with 2D materials

“…This indicates that there is an increased binding affinity, possibly from interactions with the FIB milling induced defects on the hBN flake. The nanoscopic origins of these interactions are still up for debate but a few hypotheses from molecular dynamics studies include increased electrostatic interactions and reactivity with chemical species along the defect sites 36,52,53 .…”